From this link:
http://www.microsoft.com/windowsserver2003/howtobuy/licensing/priclicfaq.mspxI have valid upgrade coverage (Upgrade Advantage or Software Assurance) on my CALs. Can I choose either User or Device CALs? How do I do that?
Yes. You must designate your Windows CALs as Device or User when you upgrade them. If you exercise your one-time right to convert Device CALs to User CALs mid-contract, you simply record the change for your records. When Software Assurance comes up for renewal, all the new specifications (for example, number of User CALs, number of Device CALs) are documented and revised in Microsoft Volume License Services' records.
For TS CALs, when you request TS CAL tokens through Terminal Server Licensing Management, you are prompted to specify User or Device.
So resuming: if you are on SA you have a one time right to convert from per device to per user.
If not, your choice is permanent.
Cláudio Rodrigues
Citrix CTP